High-density SOI cross-point memory array and method for fabricating same

ABSTRACT

A method for fabricating a high-density silicon-on-insulator (SOI) cross-point memory array and an array structure are provided. The method comprises: selectively forming a hard mask on an SOI substrate, defining memory areas, active device areas, and top electrode areas; etching to remove the exposed silicon (Si) surfaces; selectively forming metal sidewalls adjacent the hard mask; filling the memory areas with memory resistor material; removing the hard mask, exposing the underlying Si active device areas; forming an overlying layer of oxide; etching the oxide to form contact holes to the active device areas; forming diodes in the contact holes; and, forming bottom electrode lines overlying the diodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) memoryfabrication and, more particularly, to a high-density resistor randomaccess memory (RRAM) array structure and fabrication process.

2. Description of the Related Art

Conventionally, RRAMs are fabricated on bulk silicon. As is well knownin the art, there is a relatively high degree and leakage current andcapacitance associated with memory cells fabricated on bulk silicon,that translates into degraded read and write times. However,applications are beginning to demand that RRAM programming pulse widthsbe as narrow as 10 nanoseconds (ns). Silicon-on-insulator (SOI) CMOSdevices are known to have a significantly higher speed thancorresponding bulk silicon devices. Therefore, a SOI substrate RRAMwould be desirable for very high-speed memory circuits.

An SOI substrate is made from a silicon (Si) layer that overlies aninsulator material, such as sapphire or oxide. The insulation layer ofan SOI substrate completely isolates associated NMOS and PMOStransistors, to prevent the occurrence of latch-up. Further, the devicechannel doping need not overcompensate, and the diffusion regions do nothave bottom junctions. All these factor lead to a reduction in parasiticcapacitance.

In a pending application entitled, DUAL-TRENCH ISOLATED CROSSPOINTMEMORY ARRAY AND METHOD FOR FABRICATING SAME, invented by Hsu et al.,Ser. No. 10/350,643, now U.S. Pat. No. 6,875,651, filed Jan. 23, 2003,and which is incorporated herein by reference, a vertical oneresistor/one diode (1R1D) structure is described that can be used toform a high density memory array. The structure forms P+ on a highlyconductive buried N+ bit line. The resulting thickness of the P+Njunction, however, is at least 500 nanometers (nm), and not suitable forSOI processes.

FIG. 1 is a partial cross-sectional view of a dual trench isolated 1R1DRAM on bulk silicon wafer (pending art). A shallow trench extends, atleast partially, into the P+ layer to avoid the leakage current drawnfrom the bottom electrode, to the N+ bit line. The conductivity of theN+ bit line is no higher than 1 kilo-ohm (Kohm) per square if thethickness is less than 500 nm. Therefore, the minimum thickness of theSOI film has to be in the order of 500 nm in order to provide lowparasitic resistance. However, the thickness of a periphery circuit areacan be much thinner than that of the memory area, and this thicknessdifference is too large for a state-of-the-art lithograph tool tohandle.

It would be advantageous if a process were developed to increase thedensity of memory cells formed in an SOI RRAM array.

It would be advantageous if an SOI RRAM array could be formed to takeadvantage of the minimum available feature size.

SUMMARY OF THE INVENTION

The present invention describes a high-density, high-speed cross-pointresistor memory array ultra-large-scale integration (ULSI) memory chipand embedded memory application. The invention takes advantage of SOIdevice characteristics to form memory cells having a size that islimited only the feature scale.

Accordingly, a method is provided for fabricating a high-density SOIcross-point memory array. The method comprises: selectively forming ahard mask on an SOI substrate, defining memory areas, active deviceareas, and top electrode areas; etching to remove the exposed silicon(Si) surfaces; selectively forming metal sidewalls adjacent the hardmask; filling the memory areas with memory resistor material; removingthe hard mask, exposing the underlying Si active device areas; formingan overlying layer of oxide; etching the oxide to form contact holes tothe active device areas; forming diodes in the contact holes; and,forming bottom electrode lines overlying the diodes.

In some aspects of the method, selectively forming metal sidewallsadjacent the hard mask includes: isotropically depositing the metal;anisotropically etching the metal to form the sidewalls with a sidewallwidth in the range of 25 to 50 nm, between memory areas and activedevice areas. Top electrode lines are also formed by this process. Inother aspects, selectively forming metal sidewalls adjacent the hardmask includes: forming an electrode layer, from a material such as Ir,Pt, Au, or Ru, adjacent the hard mask; forming a barrier layer, from amaterial such as Ti, TiN, WN, or TaN, adjacent the electrode layer,interposing the electrode layer between the barrier layer and the hardmask.

Additional details of the above-described method, and a high-density SOIcross-point memory array are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a dual trench isolated 1R1DRAM on bulk silicon wafer (pending art).

FIG. 2 is a partial cross-sectional view of the present inventionhigh-density SOI cross-point memory array.

FIG. 3 is a partial cross-sectional detailed view of memory area, whichis representative of the other memory areas.

FIG. 4 is a plan view of the active device area of FIG. 3.

FIG. 5 is a schematic representation of the present invention arraystructure.

FIG. 6 is a plan view of the SOI substrate initial etching step.

FIG. 7 is a plan view following the formation of the top electrodes andperimeter sidewalls.

FIG. 8 is a plan view following an oxide deposition process.

FIG. 9 is a partial cross-sectional view of FIG. 8.

FIG. 10 is the cross-section view of FIG. 9 following the removal of theoxide in selected (memory) areas.

FIG. 11 is a plan view of the array of FIG. 10 following the depositionof memory resistor material.

FIG. 12 is a partial cross-sectional view of FIG. 11, following a memoryresistor material CMP process.

FIG. 13 is a plan view of FIG. 12, following the removal of thehardmask.

FIG. 14 is a partial cross-sectional view of FIG. 13.

FIG. 15 is a partial cross-sectional view following an oxide depositionstep.

FIG. 16 is a partial cross-sectional view following a contact holeetching process.

FIG. 17 is a partial cross-sectional view after the formation of diodes.

FIG. 18 is a flowchart illustrating the present invention method forfabricating a high-density SOI cross-point memory array.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a partial cross-sectional view of the present inventionhigh-density SOI cross-point memory array. The array 200 comprises a SOIsubstrate 202, including an insulator layer 204. As shown, most of theSi of the shown SOI substrate 202 has been etched away. Memory etchedareas and top electrode etched areas are formed in the SOI substrate 202(below the dotted line), extending to the insulator layer 204. Memoryresistor material 222 overlies the memory etched areas to form memoryareas 206/208/210/212. The memory resistor material 222 can be amaterial such as Pr_(0.3)Ca_(0.7)MnO₃ (PCMO), colossal magnetoresistance(CMR), or a high temperature superconductivity (HTSC) material.

SOI top layer Si active device area 224 is adjacent memory areas 206 and208, and active device area 226 is adjacent memory areas 210 and 212.Metal sidewall perimeter 228 is between the memory area 206 and activedevice area 224. Metal sidewall perimeter 230 is between the memory area208 and active device area 224. Metal sidewall perimeter 232 is betweenthe memory area 210 and active device area 226. Metal sidewall perimeter234 is between the memory area 212 and active device area 226. Alsoshown are metal sidewall top electrode lines 214, 216, 218, and 220.

An oxide passivation layer 244 overlies the memory areas 206/208/210/212and top electrode lines 214/216/218/220. Bottom electrode lines overliethe oxide passivation layer 244. Bottom electrode line 246 is shown. Adiode 248 is connected between the bottom electrode line 246 and themetal sidewall perimeters 228 and 230. Diode 250 is connected betweenthe bottom electrode line 246 and the metal sidewall perimeters 232 and234.

FIG. 3 is a partial cross-sectional detailed view of memory area 206,which is representative of the other memory areas. In some aspects, asrepresented by top electrode 214, the metal sidewall perimeters(228/230/232/234, see FIG. 2) and metal sidewall top electrode lines(214/216/218/220) have a width 300 in the range of 25 to 50 nanometers.

In other aspects, the metal sidewall perimeters and metal sidewall topelectrode lines, as represented by top electrode line 214, include abarrier layer 302 and an electrode layer 304. The barrier layer 302horizontally overlies the electrode layer 304. Alternately stated, thebarrier layer 302 is interposed between the electrode layer 304 and thememory area 206. Typically, the electrode layer 304 is formed adjacent ahard mask (as explained in detail below) and the barrier layer 302 isformed thereafter. The barrier layer 302 can be a material such as Ti,TiN, WN, or TaN. The electrode layer 304 can be a metal such as Ir, Pt,Au, or Ru. Note that metal sidewall perimeter 228 can also be formedfrom a barrier layer 302 interposed between the electrode layer 304 andthe memory area 206.

The active device areas, as represented by active device area 224, havea 1F² geometry. As shown, the active device area 224 has a width 306equal to F, where F is the minimum feature size. The active device area224 also has a length of F, extending “into the sheet” which cannot beseen in this figure. Likewise, the diodes, as represented by diode 248,have a 1F² geometry and connect with metal sidewall perimeters. Diode248 is shown connected to sidewall perimeter 228 and 230. Diode 248 isoperatively connected to memory area 206 through metal sidewallperimeter 228, and operatively connected to memory area 208 throughmetal sidewall perimeter 230. As used herein, “operatively connected”means indirectly connected or connected through an intervening element.

FIG. 4 is a plan view of the active device area 224 of FIG. 3. Thisdrawing shows that each active device area, as represented by activedevice area 224, is surrounded by metal sidewall perimeters. Shown aremetal sidewall perimeters 228, 230, 400, and 402. As explained in thefabrication process below, the four perimeters are actually formed as asingle element. The active device area 224 is formed into 1F² geometrysquares, where length 404 is equal to width 306. The memory areas, ormemory cell areas, such as memory area 206, can be formed into 4F²geometry squares.

Returning to FIG. 2, each diode includes a P++/N+ junction. Diode 248includes P++ region 251 and N+ region 252. Likewise, diode 250 includesP++ region 254 and N+ region 256. Then, the bottom electrode line 246 isa word lines and the top electrode lines 214/216/218/220 are bit lines.Alternatively the bottom electrode line 246 is a bit lines and the topelectrode lines 214/216/218/220 are word lines. Alternately but notshown, each diode could be formed with an N++/P+ junction.

FIG. 5 is a schematic representation of the present invention arraystructure. Although a 6 bit (B) by 4 word (W) array is specificallyshown, in other aspects, the word and bit lines can be exchanged. Thedesignation of lines as either word or bit lines in dependent upon thevoltage polarities applied to the finished device in operation.Considering both FIGS. 2 and 5, each diode is connected between a bottomelectrode line and a plurality of memory areas. More specifically, eachdiode is connected to the metal sidewall perimeters of a pair ofadjacent memory areas. For example, diode 248 is to adjacent memoryareas 206 (R5) and 208 (R6).

Functional Description

A cross-sectional view of the present invention SOI 1R1D RRAM structureis shown in FIG. 2. Shown is a common word line configuration, however,a common bit line configuration is equally practical. The bit lines areformed with a sidewall metal line overlying the SOI insulator. The bitline also serves as the top electrode of the RRAM memory cell. The topelectrode of the memory resistor is also a sidewall metal line oninsulator. Two adjacent resistor memory cells are connected to a SOI P+layer, which in turn, is connected to the word line through a shallow N+junction. The cell size can be as small as 4F².

FIG. 6 is a plan view of the SOI substrate initial etching step. A layerof hard mask 600, of either silicon nitride or polysilicon, is depositedonto a SOI wafer 602. The thickness of the SOI film is not critical.Photoresist is used to etch the hard mask and the SOI film, as shown.

FIG. 7 is a plan view following the formation of the top electrodes andperimeter sidewalls. RRAM metal electrode material is deposited onto thefilm and anisotropically (plasma) etched. In some aspects, a barrier,such as Ti, TiN, WN, or TaN, layer is required. The electrode metal canbe Pt or Ir, for example.

FIG. 8 is a plan view following an oxide deposition process. A layer ofoxide 800, at least 1.5 times thicker than the sum of the thickness ofthe SOI film and the hard mask 600, is depoited. After deposition, theoxide is planarized by a CMP process.

FIG. 9 is a partial cross-sectional view of FIG. 8. This view shows theSi layer 900 of the SOI substrate that has been protected by theoverlying hardmask 600.

FIG. 10 is the cross-section view of FIG. 9 following the removal of theoxide in selected (memory) areas. Photoresist is used as mask.

FIG. 11 is a plan view of the array of FIG. 10 following the depositionof memory resistor material 1100. The memory resistor material can bedeposited by spin coating, sputtering, or a MOCVD process.

FIG. 12 is a partial cross-sectional view of FIG. 11, following a memoryresistor material CMP process.

FIG. 13 is a plan view of FIG. 12, following the removal of thehardmask. The hardmask is removed by any state-of-the-art process, suchas a wet etch to remove a nitride hardmask, or a dry etch to remove apolysilicon hardmask. A photoresist mask is used to remove SOI silicon900 along the bit lines.

FIG. 14 is a partial cross-sectional view of FIG. 13.

FIG. 15 is a partial cross-sectional view following an oxide depositionstep. A layer of passivation silicon oxide 1500 is deposited and a CMPprocess performed.

FIG. 16 is a partial cross-sectional view following a contact holeetching process. The bit contact is located at the center of the squareas is shown. For minimum geometry layout this square is 1F². As aresult, the bit contact holes overlaps the metal perimeters 700.

FIG. 17 is a partial cross-sectional view after the formation of diodes.After the bit contact hole is opened, a layer of silicon is epitaxially(selectively) grown in the bit contact holes, followed by deep N+implant 1700 and a shallow P++ implant 1702, to form a P++/N+ junction.The P++/N+ depletion region does not touch metal sidewall perimeters.Alternately, the selective epitaxial grow process may be replaced withpolysilicon deposition and a SPE process. The SPE (Solid PhaseEpitaxial) growth process includes an annealing process at temperature450° C. to 600° C. for 30 minutes to 2 hours. The polysilicon is thenetched, or CMP planarized, followed by deep N+ and shallow P++ ionimplantation.

An interconnect metal is then deposited and the structure of FIG. 2results. Although the bottom electrode has been described as a wordline, and the sidewall top electrodes as bit line, the bottom and topelectrodes can be bit lines or word lines, respectively, in otheraspects of the invention. In either case, it is preferred also toarrange the polarity of the N++/P+ junction to P++/N+ junction forpositive voltage operation.

FIG. 18 is a flowchart illustrating the present invention method forfabricating a high-density SOI cross-point memory array. Although themethod is depicted as a sequence of numbered steps for clarity, no ordershould be inferred from the numbering unless explicitly stated. Itshould be understood that some of these steps may be skipped, performedin parallel, or performed without the requirement of maintaining astrict order of sequence. The method starts at Step 1800.

Step 1802 selectively forms a hard mask on an SOI substrate, definingmemory areas, active device areas, and top electrode lines. Step 1804etches to remove the exposed silicon (Si) surfaces. Step 1806selectively forms metal sidewalls adjacent the hard mask. Step 1808fills the memory areas with memory resistor material. Step 1810 removesthe hard mask, exposing the underlying Si active device areas. Step 1812forms an overlying layer of oxide. Step 1814 etches the oxide to formcontact holes to the active device areas. Step 1816 forms diodes in thecontact holes. Step 1818 forms bottom electrode lines overlying thediodes. Step 1820 forms a bottom electrode/top electrode memory array.

In some aspects of the method, selectively forming a hard mask on an SOIsubstrate in Step 1802 includes forming a hard mask from a material suchas nitride and polysilicon.

In other aspects, selectively forming metal sidewalls adjacent the hardmask in Step 1806 includes substeps (not shown). Step 1806 aisotropically deposits the metal. Step 1806 b anisotropically etches themetal to form the sidewalls between memory areas and active deviceareas, as well as top electrode lines. In some aspects, Step 1806 b usesa plasma etch process.

In one aspect, isotropically depositing the metal in Step 1806 aincludes isotropically depositing a metal thickness in the range of 50to 100 nanometers (nm). Anisotropically etching the metal in Step 1806 bincludes forming metal sidewalls and top electrode lines having asidewall width in the range of 25 to 50 nm.

In other aspects, selectively forming metal sidewalls adjacent the hardmask in Step 1806 includes alternate substeps (not shown). Step 1806 cforms an electrode layer adjacent the hard mask. Step 1806 d forms abarrier layer horizontally overlying the electrode layer. Alternatelystated, Step 1806 d interposes the electrode layer between the barrierlayer and the hard mask. In some aspects, Step 1806 c forms a barrierlayer from a metal such as Ti, TiN, WN, or TaN. Step 1806 d may includeforming an electrode layer from a metal such as Ir, Pt, Au, or Ru.

In some aspects, filling the memory areas with memory resistor materialin Step 1808 includes substeps (not shown). Step 1808 a isotropicallydeposits a layer of oxide. Step 1808 b CMP planarizes the oxide to thelevel of the hard mask. Step 1808 c etches the oxide from the memoryarea. Step 1808 d isotropically deposits memory resistor material. Step1808 e CMP planarizes the memory resistor material to the level of thehard mask. Step 1808 d may include isotropically depositing memoryresistor material by a process such as spin coating, sputtering, or ametal organic chemical vapor deposition (MOCVD) processes. In someaspects, Step 1808 fills the memory areas with memory resistor materialsuch as PCMO, colossal magnetoresistance (CMR), or a high temperaturesuperconductivity (HTSC) material.

In some aspects, removing the hard mask in Step 1810 includes substeps(not shown). Step 1810 a etches to remove all the hard mask, exposingthe underlying Si. Step 1810 b photoresist masks the Si active deviceareas. Step 1810 c etches to remove the exposed Si between adjacent topelectrode lines.

In some aspects, selectively forming a hard mask on an SOI substrate inStep 1802 includes forming 1F² geometry active device areas. Likewise,etching the oxide to form contact holes to the active device areas inStep 1814 includes substeps (not shown). Step 1814 a centers the contactholes overlying the center of the active device areas. Step 1814 b formsthe contact holes using 1F² geometry etching techniques. Step 1814 c, inresponse to forming the contact holes, exposes the metal sidewallsadjacent the active device areas.

Forming diodes in the contact holes in Step 1816 typically includesforming a diode between a bottom electrode line and a memory area in aseries of substeps (not shown). Step 1816 a epitaxially grows Si in thecontact holes. Step 1816 b performs a deep N+ implant. Step 1816 cperforms a shallow P++ implant. Step 1816 d, in response to theimplants, forms a P++/N+ junction in the Si between a bottom electrodeword line and a metal electrode sidewall adjacent a memory area.Alternately, Step 1816 b performs a deep P+ implant and Step 1816 cperforms a shallow N++ implant. Then, Step 1816 d forms a N++/P+junction in the Si between a bottom electrode bit line and a metalelectrode sidewall adjacent a memory area.

In another aspect, different substeps (not shown) can be performed. Step1816 e isotropically deposits polysilicon. Step 1816 f performs a solidphase epitaxial growth process. Step 1816 g CMP planarizes the Si to thelevel of the oxide. Step 1816 h performs a deep N+ implant. Step 1816 iperforms a shallow P++ implant. Step 1816 j, in response to theimplants, forms a P++/N+ junction in the Si between a bottom electrodeword line and a metal electrode sidewall adjacent a memory area.Alternately, Step 1816 h performs a deep P+ implant and Step 1816 iperforms a shallow N++ implant. Then, Step 1816 j forms a N++/P+junction in the Si between a bottom electrode bit line and a metalelectrode sidewall adjacent a memory area.

In other aspects, forming a diode between the bottom electrode line andthe active device areas in Step 1816 includes forming a diode between abottom electrode line and a plurality of memory areas, through a singleintervening diode. In one example, a diode is connected to the metalsidewall perimeters of a pair of adjacent memory areas.

A high-density SOI cross-point array and fabrication process has beenpresented. Examples have been used to illustrate some principles of thestructure. Likewise, example processes have been used to describefabrication. However, the invention is not limited to merely theseexamples. Other variations and embodiments of the invention will occurto those skilled in the art.

1. A method for fabricating a high-density silicon-on-insulator (SOI)cross-point memory array, the method comprising: selectively forming ahard mask on an SOI substrate, defining memory areas, active deviceareas, and top electrode areas; etching to remove exposed silicon (Si)surfaces; selectively forming metal sidewalls adjacent the hard mask;filling the memory areas with memory resistor material; removing thehard mask, exposing the underlying Si active device areas; forming anoverlying layer of oxide; etching the oxide to form contact holes to theactive device areas; forming diodes in the contact holes; forming bottomelectrode lines overlying the diodes; and, wherein forming diodes in thecontact holes includes forming a diode between a bottom electrode lineand a plurality of memory areas, through a single intervening diode. 2.The method of claim 1 wherein selectively forming a hard mask on an SOIsubstrate includes forming a hard mask from a material selected from thegroup including nitride and polysilicon.
 3. The method of claim 1wherein filling the memory areas with memory resistor material includesusing a memory resistor material selected from the group including PCMO,colossal magnetoresistance (CMR), and a high temperaturesuperconductivity (HTSC) material.
 4. The method of claim 1 whereinremoving the hard mask includes: etching to remove all the hard mask,exposing the underlying Si; photoresist masking the Si active deviceareas; etching to remove the exposed Si between adjacent top electrodelines.
 5. The method of claim 1 wherein selectively forming a hard maskon an SOI substrate includes forming 1F² geometry active device areas;and, wherein etching the oxide to form contact holes to the activedevice areas includes: centering the contact holes overlying the centerof the active device areas; forming the contact holes using 1F² geometryetching techniques; and, in response to forming the contact holes,exposing the metal sidewalls adjacent the active device areas.
 6. Themethod of claim 1 wherein forming a diode between the bottom electrodeline and a plurality of memory areas includes: epitaxially growing Si inthe contact holes; performing a deep N+ implant; performing a shallowP++ implant; and, in response to the implants, forming a P++/N+ junctionin the Si between a bottom electrode word line and a metal electrodesidewall adjacent a memory area.
 7. The method of claim 1 whereinforming a diode between the bottom electrode line and a plurality ofmemory areas includes: isotropically depositing polysilicon; performinga solid phase epitaxial growth process; CMP planarizing the Si to thelevel of the oxide; performing a deep N+ implant; performing a shallowP++ implant; and, in response to the implants, forming a P++/N+ junctionin the Si between a bottom electrode word line and a metal electrodesidewall adjacent a memory area.
 8. The method of claim 1 whereinforming a diode between the bottom electrode line and a plurality ofmemory areas includes: epitaxially growing Si in the contact holes;performing a deep P+ implant; performing a shallow N++ implant; and, inresponse to the implants, forming a N++/P+ junction in the Si between abottom electrode bit line and a metal electrode sidewall adjacent amemory area.
 9. The method of claim 1 wherein forming a diode betweenthe bottom electrode line and a plurality of memory areas includes:isotropically depositing polysilicon; performing a solid phase epitaxialgrowth process; CMP planarizing the Si to the level of the oxide;performing a deep P+ implant; performing a shallow N++ implant; and, inresponse to the implants, forming a N++/P+ junction in the Si between abottom electrode bit line and a metal electrode sidewall adjacent amemory area.
 10. The method of claim 1 wherein filling the memory areaswith memory resistor material includes: isotropically depositing a layerof oxide; and, chemical mechanically polish (CMP) planarizing the oxideto the level of the hard mask; etching the oxide from the memory area;isotropically depositing memory resistor material; and, CMP planarizingthe memory resistor material to the level of the hard mask.
 11. Themethod of claim 10 wherein isotropically depositing memory resistormaterial includes depositing memory resistor material by a processselected from the group including spin coating, sputtering, and metalorganic chemical vapor deposition (MOCVD) processes.
 12. The method ofclaim 1 wherein selectively forming metal sidewalls adjacent the hardmask includes: isotropically depositing the metal; anisotropicallyetching the metal to form the sidewalls between memory areas and activedevice areas, as well as bottom electrode lines.
 13. The method of claim12 wherein isotropically depositing the metal includes isotropicallydepositing a metal thickness in the range of 50 to 100 nanometers (nm);and, wherein anisotropically etching the metal includes forming metalsidewalls and bottom electrode lines having a sidewall width in therange of 25 to 50 nm.
 14. The method of claim 12 wherein anisotropicallyetching the metal includes using a plasma etch process.
 15. The methodof claim 1 wherein selectively forming metal sidewalls adjacent the hardmask includes: forming an electrode layer adjacent the hard mask; and,interposing the electrode layer between a barrier layer and the hardmask.
 16. The method of claim 15 wherein forming a barrier layerincludes forming a barrier layer from a metal selected from the groupincluding Ti, TiN, WN, and TaN.
 17. The method of claim 15 whereinforming an electrode layer includes forming an electrode layer from ametal selected from the group including Ir, Pt, Au, and Ru.
 18. Themethod of claim 1 wherein forming a diode between a bottom electrodeline and a plurality of memory areas, through a single intervening diodeincludes forming a diode connected to the metal sidewall perimeters of apair of adjacent memory areas.
 19. The method of claim 18 furthercomprising: forming a bottom electrode/top electrode memory array. 20.The method of claim 18 wherein the pair of adjacent memory areas form aunit memory in a cross-point memory array.
 21. A method for fabricatinga high-density silicon-on-insulator (SOI) cross-point memory array, themethod comprising: selectively forming a hard mask on an SOI substrate,defining memory areas, active device areas, and top electrode areas;etching to remove exposed silicon (Si) surfaces; selectively formingmetal sidewalls adjacent the hard mask; filling the memory areas withmemory resistor material; removing the hard mask, exposing theunderlying Si active device areas; forming an overlying layer of oxide;etching the oxide to form contact holes to the active device areas;forming diodes in the contact holes, including a single interveningdiode in a contact hole between a bottom electrode line and a pluralityof memory areas; and, forming bottom electrode lines overlying thediodes.